U.S. patent number 5,124,207 [Application Number 07/511,285] was granted by the patent office on 1992-06-23 for magnetic iron oxide particles.
This patent grant is currently assigned to Toda Kogyo Corp.. Invention is credited to Kazuyuki Hayashi, Hiroko Itamochi, Keisuke Iwasaki, Yasuyuki Tanaka.
United States Patent |
5,124,207 |
Hayashi , et al. |
June 23, 1992 |
Magnetic iron oxide particles
Abstract
Disclosed herein is a magnetic iron oxide particle comprising a
magnetic iron oxide core particle containing ferrous iron, a Zn
compound layer as a lower coating layer on the surface of said core
particle, and an Si compound layer as an upper coating layer
thereon.
Inventors: |
Hayashi; Kazuyuki (Hiroshima,
JP), Iwasaki; Keisuke (Hiroshima, JP),
Tanaka; Yasuyuki (Hiroshima, JP), Itamochi;
Hiroko (Hiroshima, JP) |
Assignee: |
Toda Kogyo Corp. (Hiroshima,
JP)
|
Family
ID: |
14346090 |
Appl.
No.: |
07/511,285 |
Filed: |
April 10, 1990 |
Foreign Application Priority Data
|
|
|
|
|
Apr 20, 1989 [JP] |
|
|
1-103135 |
|
Current U.S.
Class: |
428/404;
252/62.58; 252/62.59; 252/62.62; 252/62.63; 252/62.64; G9B/5.269;
G9B/5.276 |
Current CPC
Class: |
G11B
5/70689 (20130101); G11B 5/712 (20130101); Y10T
428/2993 (20150115) |
Current International
Class: |
G11B
5/706 (20060101); G11B 5/712 (20060101); C04B
035/26 () |
Field of
Search: |
;428/403,404
;252/62.58,62.59,62.62,62.63,62.64 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4082905 |
April 1978 |
Stephan et al. |
4259368 |
March 1981 |
Rudolf et al. |
4956220 |
September 1990 |
Sueyoshi et al. |
|
Foreign Patent Documents
Primary Examiner: Cooper; Jack
Attorney, Agent or Firm: Nixon & Vanderhye
Claims
What is claimed is:
1. An acicular magnetic iron oxide particle comprising a
Co-modified acicular magnetic iron oxide core particle containing
ferrous, a Zn compound layer of 0.1 to 5% by weight, calculated as
Zn, based on the Co-modified acicular magnetic iron oxide core
particle containing ferrous, as a lower coating layer on the
surface of said core particle, and an Si compound layer of 0.1 to
5% by weight, calculated as SiO.sub.2, based on the Co-modified
acicular magnetic iron oxide core particle containing ferrous, as
an upper coating layer thereon, wherein
the weight ratio of the Zn compound layer to Si compound layer
being 0.2-5/1, and the acicular magnetic iron oxide particle
has a major axial diameter of 0.05 to 0.35 .mu.m, an axial ratio
(major axial diameter/minor axial diameter) of not less than 5.0, a
coercive force of 600 to 1200 Oe and exhibits a smaller percent
reduction in Fe.sup.2+ content after seven days in air at
60.degree. C. and 90% RH than said acicular magnetic iron oxide
particle in which either the Zn compound layer or the Si compound
layer is absent or where the Zn compound is the upper coating layer
and the Si compound is the lower coating layer on the surface of
said core particle.
2. An acicular magnetic iron oxide particle according to claim 1,
wherein the amount of the Zn compound layer and the Si compound
layer is not more than 6% by weight based on the Co-modified
acicular magnetic iron oxide particle containing ferrous.
3. An acicular magnetic iron oxide particle according to claim 1,
which exhibits a reduction rate of coercive force after seven days
in air at 60.degree. C. and 90% RH of not more than 2%.
4. An acicular magnetic iron oxide particle according to claim 1,
which exhibits a reduction rate of Fe.sup.2+ after seven days in
air at 60.degree. C. and 90% RH of less than 20% by weight.
5. An acicular magnetic iron oxide particle comprising a
Co-modified acicular magnetic iron oxide core particle containing
ferrous, a Zn compound layer of 0.1 to 5% by weight, calculated as
Zn, based on the Co-modified acicular magnetic iron oxide core
particle containing ferrous, as a lower coating layer on the
surface of said core particle, and a layer of compounds containing
Si and at least one metal selected from Al, Ca, Zr, Sb, Ti, V, Mg,
Ba and Zn, of 0.05 to 5% by weight, calculated as SiO.sub.2 plus
said metal element(s), based on the Co-modified acicular magnetic
iron oxide core particle containing ferrous, as an upper coating
layer thereon, wherein
the weight ratio of the Zn compound layer to the compound layer
containing Si and said metal(s) is 0.2-5/1 and the molar ratio of
the Si compound to said metal(s) compound of the upper layer is
1-10/1, and the acicular iron oxide particle has a major axial
diameter of 0.05 to 0.35 .mu.m, an axial ratio (major axial
diameter/minor axial diameter) of not less than 5.0, a coercive
force of 600 to 1200 Oe and exhibits a smaller percent reduction in
Fe.sup.2+ content after seven days in air at 60.degree. C. and 90%
RH than said acicular magnetic iron oxide particle in which either
the Zn compound layer or the layer of compounds containing Si and
said at least one metal is absent or where the Zn compound is the
upper coating layer and the compounds containing Si and said at
least one metal are the lower coating layer on the surface of said
core particle.
6. An acicular magnetic iron oxide particle according to claim 5,
wherein the amount of the Zn compound and the compounds layer
containing Si and said metal is not more than 6% by weight based on
the Co-modified acicular magnetic iron oxide particle containing
ferrous.
7. An acicular magnetic iron oxide particle according to claim 5,
which exhibits a reduction rate of coercive force after seven days
in air at 60.degree. C. and 90% RH of not more 2%.
8. An acicular magnetic iron oxide particle according to claim 5,
which exhibits a reduction rate of Fe.sup.2+ after seven days in
air at 60.degree. C. and 90% RH of less than 20% by weight.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the magnetic iron oxide particles
containing ferrous, which the particles are fine in size, high in
coercive force, and magnetically and chemically stable.
With progressing miniaturization and weight reduction of magnetic
recording and reproducing devices in recent years, the necessity
has arisen keenly for the higher quality and performance of
recording media such as magnetic tapes and discs. Especially
demands are heard for higher recording density, higher sensitivity,
higher output capacity, and higher magnetic and chemical stability
of the magnetic media.
For satisfying such requirements for the magnetic recording media,
the magnetic iron oxide particles used for making such recording
media are essentially demanded to be fine in size, high in coercive
force, and also magnetically and chemically stable.
This fact is referred to in many literatures. For instance,
"Development of Magnetic Materials and High Dispersion Techniques
for Magnetic Powders" (1982) published by Sogo Gijutsu Center Co.,
Ltd. states on page 310: "Since the main themes of improvement on
magnetic tape performance were the realization of higher
sensitivity, higher output capacity and lower noise level, the
studies have been directed to the attainment of higher coercive
force and smaller size of acicular .gamma.-Fe.sub.2 O.sub.3
particles". Also, Japanese Patent Publication No. 55-6580 states:
"There is seen a tendency of recording signals to transfer to the
shorter wavelength region in recent years, such a tendency being
particularly conspicuous in video cassette recorders. In other
words, demands is rising for higher-density recording,
higher-output capacity and, in particular, improvement of frequency
characteristics and magnetic stability. The properties of the
magnetic material for satisfying these requirements for the
magnetic recording media are magnetic stability and a high coercive
force (Hc)".
Among the known magnetic iron oxide particles, magnetite particles
have the advantages that they are high in coercive force and
saturated flux density in comparison with maghemite particles, and
that when a magnetic recording medium is made by using magnetite
particles, such recording medium is hardly electrically charged
because of high electroconductivity.
As a typical example of magnetic iron oxide particles having high
coercive force, there are known so-called Co-modified magnetic iron
oxide particles which have been produced by coating on or
substituting the surfaces of magnetite or maghemite particles as
precursor particles with a Co compound. In preparation of such
Co-modified magnetic iron oxide particles, it is commonly practiced
to incorporate ferrous along with Co in the particles, when coating
on or substituting their surfaces with a Co compound, for further
enhancing the coercive force.
Thus, the magnetic iron oxide particles which are fine in size,
high in coercive force, and also magnetically and chemically stable
are the most eagerly required in the art at present. However, known
magnetite particles or Co-modified magnetic iron oxide particles,
although having high coercive force as mentioned above, have the
defect that they are magnetically and chemically unstable due to
incorporation of ferrous. That is, when the magnetic iron oxide
particles containing ferrous are left in the air, ferrous contained
therein is oxidized into ferric, thereby deteriorating magnetic
properties, especially reduction of coercive force, with the
passage of time. This phenomenon tends to become more marked as the
particle size becomes smaller.
Recently, the problem is also pointed out that when magnetic iron
oxide particles containing ferrous are used as magnetic tape
coating material, the color tone of the coat is changed from normal
black into dark brown due to oxidation of ferrous in the said
particles into ferric to cause an increase of light transmittance,
resulting in inducing improper run of magnetic tape in the
recording devices such as video deck.
As a result of the present inventors ' extensive studies to provide
magnetic iron oxide particles containing ferrous which are free of
the said defects and can well satisfy the said requirements, it has
been found that magnetic iron oxide particles containing ferrous
and having their surfaces coated with double layers of a lower
layer of a Zn compound and an upper layer of a Si compound, which
have been obtained by mixing and dispersing magnetic iron oxide
particles containing ferrous in a solution of a Zn compound to
adsorb the Zn compound on the particle surfaces, and then mixing
and dispersing the obtained Zn compound-adsorbed particles in a
solution of a Si compound to adsorb the Si compound on the Zn
compound-adsorbed particle surfaces, are satisfactorily fine in
size, high in coercive force, and also magnetically and chemically
stable. The present invention was achieved on the basis of this
finding.
SUMMARY OF THE INVENTION
In a first aspect of the present invention, there is provided a
magnetic iron oxide particle comprising a magnetic iron oxide core
particle containing ferrous, a Zn compound layer as a lower coating
layer on the surface of the said core particle, and an Si compound
layer as an upper coating layer thereon.
In a second aspect of the present invention, there is provided a
magnetic iron oxide particle comprising a magnetic iron oxide core
particle containing ferrous, a Zn compound layer as a lower coating
layer on the surface of the said core particle, and a layer of
compounds containing Si and at least one element selected from Al,
Ca, Zr, Sb, Ti, V, Mg, Ba and Zn, as an upper coating layer
thereon.
BRIEF EXPLANATION OF THE DRAWING
FIG. 1 is a graph showing change in content of ferrous iron in the
Co-modified magnetite particles when the said particles were left
under the conditions of 60.degree. C. and 90% RH. In the graph, A
to F represent the patterns of change in the Co-modified magnetite
particles of Example 2, Example 1, Comparative Example 7,
Comparative Example 4, Comparative Example 3 and Comparative
Example 1, respectively.
DETAILED DESCRIPTION OF THE INVENTION
The term "magnetic iron oxide particles containing ferrous" used in
the present invention means magnetite (FeO.sub.x .multidot.Fe.sub.2
O.sub.3, O<.times..ltoreq.1) particles, and Co-modified
magnetite or maghemite particles having their surfaces coated or
modified with Co and Fe (II).
The Zn compound of the lower layer on the surface of the magnetic
iron oxide particles according to the present invention includes
zinc sulfate, zinc nitrate, zinc chloride, zinc acetate, zinc
oxides, zinc hydroxides and the like. As the content of Zn compound
in the particle surface, 0.1 to 5% by weight, calculated as Zn,
based on the magnetic iron oxide particle containing ferrous iron
are preferred. When this content is less than 0.1% by weight, the
desired effect tends not to be obtained. On the other hand, when
the content exceeds 5% by weight, although there can be obtained
the magnetically and chemically stable magnetic iron oxide
particles, saturation magnetization of the particles tends to lower
due to the increase of Zn compound which has no concern with
magnetism of the particle.
The Si compound of the upper layer on the surface of the magnetic
iron oxide particle according to the present invention includes
water glass, colloidal silica, silicon oxide, sodium silicate,
potassium silicate and the like. As the content of Si compound in
the particle surface 0.1 to 5% by weight, calculated as SiO.sub.2,
based on the magnetic iron oxide particle containing ferrous are
preferred. When the content of Si compound is less than 0.1% by
weight, the desired effect tends not to be obtained. When the
content exceeds 5% by weight, although there can be obtained the
magnetically and chemically stable magnetic iron oxide particles,
saturation magnetization tends to lower because of the increase of
Si compound which has no concern with magnetism of the
particle.
The weight ratio of Zn compound layer to Si compound layer in the
present invention is in the range of 0.2-5/1. When this ratio is
less than 0.2/1 or exceeds 5/1, it becomes hard to obtain the
desired effect.
The Si compound layer (upper coating layer) on the surface of the
particle according to the present invention may be a layer of
compounds containing Si and at least one metal selected from the
group consisting of Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn.
The compounds of metals Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn usable
in the present invention are the sulfates, nitrates, chlorides,
oxides, hydroxides, etc., of the said metals. The total content of
the compounds of Si and at least one metal selected from Al, Ca,
Zr, Sb, Ti, V, Mg, Ba and Zn on the surface of the magnetic iron
oxide particle is 0.05 to 5% by weight, calculated as the sum of
the amount of SiO.sub.2 and the amount of specified metal
element(s), based on the magnetic iron oxide particle containing
ferrous. When the content is less than 0.05% by weight, the desired
effect tends not to be obtained, and when the content exceeds 5% by
weight, although there can be obtained the magnetically and
chemically stable magnetic iron oxide particles, saturation
magnetization of the particle tends to lower due to the increase of
Si and specified metal(s) which has no concern with magnetism of
the particle. The Si compound/specific metal ratio in the upper
layer is, expressed as SiO.sub.2 /specific metal element molar
ratio, is 1-10/1. When this ratio is less than 1/1 or exceeds 10/1,
it is difficult to obtain the desired magnetically and chemically
stable magnetic iron oxide particles containing ferrous.
The total amount of Zn compound of the lower layer and Si compound
or compounds containing Si and at least one metal selected from Al,
Ca, Zr, Sb, Ti, V, Mg, Ba and Zn of the upper layer is preferably
not more than 6% by weight, calculated as the sum of the amount of
SiO.sub.2 and the amount of metal element(s), based on the weight
of the magnetic iron oxide particle containing ferrous. When the
total amount of the compounds of the lower and upper layers exceeds
6% by weight, although there can be obtained the chemically stable
magnetic iron oxide particles, saturation magnetization of the
particles tends to lower due to the increase of non-magnetic
compounds in the particle surface.
As seen from the Examples and Comparative Examples described later,
it has been confirmed that the desired effect of the present
invention cannot be obtained either in case of coating the particle
surface with a single Zn compound layer or Si compound layer, or in
case of coating the particle surface with double layers in which
the lower layer is an Si compound layer and the upper layer is a Zn
compound layer. However, there can be obtained a magnetically and
chemically stable magnetic iron oxide particle containing ferrous
when the particle surface is coated with double layers in which the
lower layer is a Zn compound layer and the upper layer is an Si
compound layer.
FIG. 1 is a graph showing the change of content of ferrous in the
Co-modified magnetite particles when they were left under the
conditions of 60.degree. C. and 90% RH. In the graph, curves A and
B represent the change of ferrous content in the Co-modified
magnetite particles obtained in Example 2 and Example 1,
respectively; curve E represents the change of ferrous content in
the Co-modified magnetite particles obtained in Comparative Example
3, and curve D represents the change of ferrous content in the
Co-modified magnetite particles obtained in Comparative Example
4.
As seen from the FIG. 1, the Co-modified magnetite particles having
a double-layer coating comprising a lower layer composed of a zinc
hydroxide and an upper layer composed of silicon oxide are very
chemically stable in comparison with the Co-modified magnetite
particles having their surfaces coated with a single layer of a
zinc hydroxide or silicon oxide.
Coating of the particle surfaces with a Zn compound layer in the
present invention can be accomplished by, for instance, the
following methods: the magnetic iron oxide particles containing
ferrous are mixed and dispersed in a solution of a Zn compound such
as zinc sulfate, zinc nitrate, zinc chloride, zinc acetate, zinc
oxides or zinc hydroxides so that the Zn compound is adsorbed on
the particle surfaces; a solution of a Zn compound such as zinc
sulfate, zinc nitrate, zinc chloride, zinc acetate or the like is
neutralized with an alkali and the magnetic iron oxide particles
containing ferrous are mixed and dispersed in the resultant
solution so that zinc hydroxides is deposited on the particle
surfaces.
Coating of the particle surfaces with an Si compound layer or a
layer of compounds containing Si and at least one metal selected
from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn in the present invention
can be accomplished by, for instance, the following methods: the
magnetic iron oxide particles containing ferrous coated with Zn
compound layer are mixed and dispersed in a solution of an Si
compound such as water glass, colloidal silica, silicon oxide,
sodium silicate potassium silicate etc., or a mixed solution of
such an Si compound and compound(s) of one or more of the said
metals such as sulfate, nitrate, chloride, oxide or hydroxide of
the said metals, so that the Si compound or the compounds
containing Si and the specified metal(s) is (are) adsorbed on the
particle surfaces; a solution of an Si compound or a mixed solution
of Si and specified metal(s) is neutralized with an acid or alkali
and the magnetic iron oxide particles containing ferrous are mixed
and dispersed in the resultant solution so that silicon oxide, or
an oxide or hydroxide containing Si and specified metal(s) is (are)
deposited on the particle surfaces.
When carrying out the coating treatment with a Zn compound in the
present invention, it is preferable to have the particles dispersed
well in the solution by previously adding a dispersing agent into
an aqueous suspension of the magnetic iron oxide particles
containing ferrous, As the dispersing agent, iron sol, alumina sol,
zirconia sol and the like can be effectively used.
The magnetic iron oxide particles according to the present
invention have the following properties: a particle shape is
acicular, spindle, cubic, octahedron, spherical or plate-like; a
particle diameter is not more than 0.40 .mu.m, preferably 0.05 to
0.35 .mu.m; in the case of acicular particles, a major axial
diameter is not more than 0.40 .mu.m, preferably 0.05 to 0.35
.mu.m, and an axial ratio (major axial diameter/minor axial
diameter) is not less than 5.0, preferably not less than 6.0; a
coercive force (Hc) in the case of acicular particles non
Co-modified is 300-400 Oe and the coercive force (Hc) in the case
of acicular particles Co-modified is 400-1200 Oe, preferably
600-900 Oe, a coercive force (Hc) in the case of cubic particles
non Co-modified is 70-110 and the coercive force (Hc) in the case
of cubic particle Co-modified is 100-900 Oe, preferably 100-600 Oe,
a coercive force (Hc) in the case of octahedron particles non
Co-modified is 110-140 Oe and the coercive force (Hc) in the case
of octahedron particles Co-modified is 150-950 Oe, preferably
150-650 Oe, a coercive force (Hc) in the case of spherical
particles non Co-modified is 40-80 Oe and the coercive force (Hc)
in the case of spherical particles Co-modified is 50-700 Oe,
preferably 50-500 Oe; reduction rate of coercive force after the
lapse of 7 days is not more than 2%, preferably not more than 1.5%;
and reduction rate of Fe.sup.2+ after the lapse of 7 days is less
than 20% by weight, preferably less than 10% by weight.
The magnetic iron oxide particles according to the present
invention are suited for use as magnetic iron oxide particles for
producing magnetic recording media with high recording density,
high sensitivity and high output capacity.
Further, since the magnetic iron oxide particles according to the
present invention are magnetically and chemically stable, and a
color tone change thereof from black into dark brown is prevented,
they can be used as magnetic iron oxide particles for
electrophography.
EXAMPLES
The present invention will be more precisely explained while
referring to Examples as follows.
However, the present invention is not restricted to Examples under
mentioned. From the foregoing description, one skilled in the art
can easily ascertain the essential characteristics of the present
invention, and without departing from the spirit and scope thereof,
can make various changes and modifications of the invention to
adapt it to various usages and conditions.
In the following descriptions of Examples and Comparative Examples,
the major axial diameter of particle and the axial ratio (major
axial diameter/minor axial diameter) were shown by the average of
the values determined from electron micrographs.
The contents of Zn, Si, Al, Ca and Co were determined by a
fluorescent X-ray analysis, and Fe.sup.2+ was measured according to
a chemical analytical method.
The magnetic properties were determined by applying an external
magnetic field of up to 10 K0e by using a sample vibrating type
magnetometer Model VSM-3S-15 manufactured by Toei Kogyo K.K.
The initial Fe.sup.2+ content in the magnetic iron oxide particles
containing ferrous was shown by the value determined from the
particles obtained after drying in high-purity nitrogen at
60.degree. C. for 24 hours. The change of Fe.sup.2+ content with
the passage of time was shown by the values determined in the air
under the conditions of 60.degree. C. and 90% RH.
EXAMPLE 1
500 g of Co-modified acicular magnetite particles having a major
axial diameter of 0.2 .mu.m, an axial ratio (major axial
diameter/minor axial diameter) of 7 : 1, an Fe.sup.2+ content of
16.7% by weight, a Co content of 4.24% by weight and a coercive
force of 850 Oe were mixed and dispersed in 5 litres of an
8.6.times.10.sup.-3 mol/1 aqueous solution of zinc chloride
(equivalent to 0.56% by weight, calculated as Zn, based on the core
particle). Then an NaOH solution was added to the resultant
dispersion to adjust its pH to 7.0, thereby forming a zinc
hydroxides layer on the particle surfaces, and then the thus
obtained particles were filtered and dried by the usual methods to
obtain black particles. The amount of Zn present on the surface of
the obtained black particle, as measured by fluorescent X-ray
analysis, was 0.55% by weight.
To an aqueous suspension containing 100 g of the said black
particles was added sodium hydroxide to adjust pH of the suspension
to 11.0, followed by addition of 6 g of #3 water glass (equivalent
to 1.74% by weight, calculated as SiO.sub.2, based on the core
particle). Then the suspension was stirred and mixed up, and
sulfuric acid was added to the resultant suspension to adjust pH to
7.0, thereby forming an SiO.sub.2 layer on the surface of the
acicular magnetite particle coated with zinc hydroxides.
The suspension containing the thus obtained sediment of black
particles was filtered, washed with water and dried in N.sub.2 gas
at 60.degree. C. for 24 hours, all in the usual methods.
The amount of SiO.sub.2 present on the thus obtained black particle
surface, as measured by fluorescent X-ray analysis, was 1.71% by
weight. The results of chemical analysis also showed that the
particle had an Fe.sup.2+ content of 16.4% by weight and a coercive
force of 846 Oe.
The change with the passage of time (of Fe.sup.2+ content and
coercive force) of the obtained black particles when left under the
environmental conditions specified above was as shown by curve B in
FIG. 1 and in Table 2, which indicates very excellent chemical and
magnetic stability of the particles.
EXAMPLES 2-4 and COMPARATIVE EXAMPLES 1-6
Coated particles were obtained by following the same procedure as
Example 1 except for the change of the kind of core particle, the
kind and amount of Zn compound, the kind and amount of Si compound
and the kind and amount of specified metal used.
The change with the passage of time (of Fe.sup.2+ content and
coercive force) of the Co-modified acicular magnetic iron oxide
particles which had no coating-treatment and of the coated
particles described above was as shown in Table 2.
Also, the change of Fe.sup.2+ content in the particles of Example
2, Comparative Example 1, Comparative Example 3 and Comparative
Example 4 when they were left under the specified environmental
conditions was as shown in FIG. 1, in which curves A, F, E and D
represent the particles of Example 2, Comparative Example 1,
Comparative Example 3 and Comparative Example 4, respectively. As
seen from Table 2 and FIG. 1, the magnetic iron oxide particles of
Examples 2 to 4 are very excellent chemical and magnetic stability.
In contrast, the magnetic iron oxide particles of Comparative
Examples 1 to 6 were very poor in chemical and magnetic
stability.
COMPARATIVE EXAMPLE 7
Coated particles were obtained in the same procedure as Example 1
except that the order of coating-treatment for forming the Zn
compound layer and the Si compound layer was reversed. The change
with the passage of time of Fe.sup.2+ content and coercive force of
these particles when left under the specified environmental
conditions is shown in Table 2.
Also, the change of Fe.sup.2+ content in the particles when left
under said conditions is represented by curve C in FIG. 1. As seen
from Table 2 and FIG. 1, the magnetic iron oxide particles of
Comparative Example 7 were poor in chemical and magnetic
stability.
TABLE 1
__________________________________________________________________________
Treatment with Zn compound Zn compound Amount Kind of particle to
be treated added, Addition Coercive calcd. as of alkali, Kind
Fe.sup.2+ (wt %) force Hc (Oe) Kind Zn (wt %) & pH
__________________________________________________________________________
EXAMPLE 1 Co-modified acicular magnetite 16.7 850 Zinc 0.56 NaOH
particle (major axis: 0.2 .mu.m; chloride 7.0 axial ratio = 7/1; Co
= 4.2 wt %) 2 Co-modified acicular magnetite 16.7 850 Zinc 0.56
NaOH particle (major axis: 0.2 .mu.m; chloride 7.0 axial ratio =
7/1; Co = 4.2 wt %) 3 Co-modified acicular maghemite 5.8 891 Zinc
0.51 NaOH particle (major axis: 0.3 .mu.m; acetate 7.0 axial ratio
= 8/1; Co = 5.20 wt %) 4 Co-modified acicular maghemite 5.8 891
Zinc 05.1 NaOH particle (major axis: 0.3 .mu.m; acetate 7.0 axial
ratio = 8/1; Co = 5.20 wt %) COMP. EXAMPLE 1 Same Co-modified
acicular 16.7 850 -- -- -- magnetite particle as used in Example 1
2 Same Co-modified acicular 5.8 891 -- -- -- maghemite particle as
used in Example 3 3 Same Co-modified acicular 16.7 850 Zinc 0.56
NaOH magnetite particle as used in chloride 7.0 Example 1 4 Same
Co-modified acicular 16.7 850 -- -- -- magnetite particle as used
in Example 1 5 Same Co-modified acicular 5.8 891 Zinc 0.51 NaOH
maghemite particle as used in acetate 7.0 Example 3 6 Same
Co-modified acicular 5.8 891 -- -- -- maghemite particle as used in
Example 3
__________________________________________________________________________
Treatment with Si compound or compounds Containing Si and specific
metal Si compound Coated particles Amount Content added, Specific
metal of spe- calcd. Amount Addition Content Content cific Coercive
as SiO.sub.2 added of acid of Zn of SiO.sub.2 metal Fe.sup.2+ force
Kind (wt %) Kind (wt %) & pH (wt %) (wt %) (wt %) (wt %) Hc
(Oe)
__________________________________________________________________________
EXAMPLE 1 #3 water 1.74 -- -- Sulfuric 0.55 1.71 -- 16.4 846 glass
acid 7.0 2 #3 water 1.74 Alu- 0.27 Sulfuric 0.54 1.69 0.26 16.4 849
glass minum acid sulfate 7.0 3 #3 water 1.45 -- -- Sulfuric 0.51
1.43 -- 5.7 892 glass acid 7.0 4 #3 water 1.45 Calcium 0.20
Sulfuric 0.50 1.42 0.19 5.6 891 glass chlo- acid ride 7.0 COMP.
EXAMPLE 1 -- -- -- -- -- -- -- -- 16.7 844 2 -- -- -- -- -- -- --
-- 5.8 890 3 -- -- -- -- -- 0.56 -- -- 16.6 843 4 #3 water 1.74 --
-- Sulfuric -- 1.72 -- 16.5 846 glass acid 7.0 5 -- -- -- -- --
0.50 -- -- 5.7 886 6 #3 water 1.45 -- -- Sulfuric -- 1.43 -- 5.7
886 glass acid 7.0
__________________________________________________________________________
TABLE 2
__________________________________________________________________________
Kind of coated particles Change with the passage of time of
Fe.sup.2+ content Change with the passage of time of coercive (wt
%) under environment of 60.degree. C. and 90% force (Oe) under
environment of 60.degree. C. and 90% RH Initial 1 day 3 days 5 days
7 days Initial 1 day 3 days 5 days 7 days
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EXAMPLE 1 16.4 16.0 15.5 15.1 14.9 846 843 843 841 841 2 16.4 16.3
16.0 15.5 15.4 849 849 846 847 846 3 5.7 5.5 5.2 5.2 5.2 892 889
886 882 882 4 5.6 5.6 5.4 5.3 5.4 891 891 886 889 889 COMP. EXAMPLE
1 16.7 13.0 10.9 9.8 8.9 844 825 800 783 768 2 5.8 4.3 3.8 3.5 3.2
890 867 838 821 796 3 16.6 13.4 11.0 10.3 9.9 843 830 810 801 779 4
16.5 14.2 12.4 11.4 10.6 846 832 828 801 796 5 5.7 5.0 4.7 4.2 4.0
886 869 849 840 831 6 5.7 5.3 5.0 4.3 4.3 886 874 867 862 859 7
16.4 15.5 14.6 14.3 13.7 845 836 830 821 810
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